The present invention relates to a method for controlling a brake system of a motor vehicle, in particular for controlling a regenerative brake system, with a number of friction brakes and an electric generator for a motor vehicle. It further relates to a regenerative brake system for a motor vehicle.
So-called regenerative brake systems can be employed in motor vehicles, wherein at least part of the energy produced during braking in the vehicle can be stored and re-used later on for the drive of the vehicle. This allows reducing the energy consumption of the vehicle in total, increasing the efficiency, and thus rendering its operation more economical. To comply with varying system requirements, motor vehicles with a regenerative brake system generally include different types of brakes, which are also called brake actuators, i.e. typically hydraulically actuated friction brakes, on the one hand, and an electro-regenerative brake, on the other hand.
Like in conventional friction brakes, the brake pressure for the friction brakes in systems of this type is generated at least partly by means of a brake pressure generating means or by way of the brake pedal movement, respectively. The electro-regenerative brake is generally configured as an electric generator, through which at least part of the total brake capacity is generated. The produced electric energy is fed into a storage medium such as an on-board battery, or fed back, respectively, and is reused for the drive of the motor vehicle by way of an appropriate drive.
Regenerative brake systems can be designed as so-called serial regenerative concepts, where the component of the brake torque that is produced by the generator is as high as possible. In contrast thereto, parallel or so-called residual-moment-based regenerative concepts are known, where the brake torque is distributed in predefined ratios to the brake actuators. Mixed concepts of these two brake concepts are also known in the art. It is common to all systems that several brake actuators are used for simultaneous braking at least in some ranges of the brake torque to be generated so that the total deceleration is composed of the deceleration components of the brake actuators.
In this arrangement, the braking energy is generally split up into components of the friction brakes and into components of the electric generator, said split-up being dependent on the nominal brake torque, the charging condition of the battery, and especially the field of operation and other special properties of the generator. Due to the split-up of braking energy, the brake pressure is therefore built up at least partly independently of the hydraulic influence of the brake pedal in regenerative brake systems.
In conventional brake systems, however, which have only one friction brake, the brake pressure is built up depending on the position of the brake pedal. In this arrangement, the pressure of a braking medium is built up by way of the position of the brake pedal with or without auxiliary energy, the braking medium being received in the friction brake. Thus, the pedal position corresponds to the brake performance of the motor vehicle. Exceptions can be the use of electronic safety systems such as an electric stability program (ESP), which can comprise devices for the independent brake pressure development irrespective of the brake pedal position.
Compared to this conventional brake performance, it is disadvantageous in the brake performance of a regenerative brake system, composed of the combination of an electro-regenerative brake and a conventional, hydraulically operated friction brake that the position of the brake pedal does not necessarily correspond to the brake performance of the motor vehicle. Hence, the position of the brake pedal can remain constant e.g. during increase of the brake deceleration, what is a very unusual brake feeling for the driver, with the result that only insufficient brake comfort develops.
So-called ‘brake-by-wire’ systems can be employed to solve this problem. Electronic sensors are used in these brake systems to sense the braking request of the driver at the brake pedal. As this occurs, brake fluid is conducted from a high-pressure accumulator to the friction brakes e.g. by way of electronically actuated valves. The high-pressure accumulator is filled with brake fluid by a motor-and-pump assembly and is acted upon by the corresponding pressure. In ‘brake-by-wire’ systems of this type, exclusively the high-pressure accumulator generally produces the required brake pressure at the friction brakes, with the exception of failure of system components, so that the brake pedal is principally uncoupled from the friction brakes because the control or the supply of brake fluid to the friction brakes is electronically controlled. Uncoupling the brake pedal from the friction brakes allows integrating a pedal control, which is appropriate for a regenerative brake system, into the brake system so that an acceptable brake pedal feeling is achieved.
When a regenerative ‘brake-by-wire’ system is designed as a serial brake system, the aim is to achieve a maximum great brake torque component by the electro-regenerative brake. To this end, braking is initially performed using the electric generator exclusively. The friction brakes are connected electronically only when the total nominal brake torque exceeds the brake torque, which can maximally be generated by the generator, as has been described in the above. The brake torque, which can be produced maximally by the generator, depends both on the charging condition of the on-board battery and on the operating range and efficiency of the generator, varying depending on the speed of the motor vehicle.
When connecting the friction brakes in a ‘brake-by-wire’ system of this type, it is disadvantageous that the friction brakes are activated and deactivated repeatedly due to variations of the maximum generator brake torque in the course of speed, or due to variations in the braking request, what causes an increased wear of the friction brakes. Due to the increased stress caused by noise in repeating connecting and disconnecting actions of friction brakes, this brake performance also causes an unpleasant braking and driving comfort for the driver. A similar problem can occur when the friction brakes are actuated separately at different axles of the motor vehicle and, due to an optimal brake force distribution and the generator brake torque, which usually acts on only one axle of the motor vehicle, the friction brakes at the other axle are activated, however, they are not activated on the generator axle. Likewise in this event, insignificant variations of the braking request or the available generator braking power render the friction brakes at the generator axle repeatedly active and passive. In this case, too, repeated connection and disconnection of the friction brakes causes a negative brake feeling for the driver.
In view of the above, an object of the invention is to disclose a method for controlling a brake system as described above, which allows achieving great braking comfort with the brake system, yet entails little effort. Another objective relates to a brake system for implementing the method.